In recent years, there has been a growing interest in developing micro/meso photonic structures with well-controlled periodic structures and well-defined defects. In particular there has been growing interest in the fabrication of photonic crystals. Photonic crystals (also known as photonic band-gap materials) are periodic dielectric structures that have a band gap that forbids propagation of a certain frequency of light. In photonic crystals three-dimensional (3D) periodic modulation in refractive index is used to cause an incident electromagnetic (EM) radiation with wavelength proportional to the periodicity of the photonic crystal to undergo Bragg diffraction in a given direction, resulting in the formation of a stop band in the transmission spectrum (see FIG. 5D). The introduction of metal or semiconductor in such periodic structures has been shown to broaden the stop band, leading to photonic band gap structures with novel application in numerous technologically important areas.
Photonic crystals permit the controlled manipulation of photons, thereby the control of electromagnetic radiation. The bandgap of the crystal can be varied by methods well known to those of ordinary skill in the art, as for example, altering the lattice spacing of the photonic material making up the crystals to match the wavelength which is wished to be blocked. That is, if the crystal is assembled in a precise lattice-like manner, the crystal can have a photonic bandgap, a range of forbidden frequencies within which a specific wavelength is blocked, and light is reflected.
Photonic crystals are typically fabricated from dielectric material, that is material that is an electrical insulator or in which an electric field can be perpetuated with a minimum loss of power. Periodic structures having a strong modulation in refractive index, and a diamond-type lattice, were among the first to be used as photonic crystals.
Many uses have been proposed in the art for photonic crystals, in particular in the fields of optoelectronics and photonics. Such uses include, but are not limited to, providing for zero threshold lasers based on the crystals ability to inhibit spontaneous emissions, use of the crystals as single-mode light emitting diodes, high Q frequency selective filters, angular filters and polarizers, use of the crystals in planar antenna substrates (thereby increasing the power radiated into the air) and solar cells, and employment of the crystals in wave guides in the optical domain, in particular as low-loss cladding material in waveguide structures that contain bends or junctions. Other applications for such crystal include their use in highly efficient reflectors (e.g. in the microwave and millimeter wave range where such structures present the ultimate substrate for antenna mounting).
As photonic crystals may be used to localize light within diffracting walls such that light can be bent without loss over tight 90° bends, to amplify light with minimum expense of energy (zero threshold lasing) and can be used to manipulate the path of light, the prospect of all-optical computing may in the near future become a reality. In addition, such structures may find important immediate applications in specialized mirror technologies, fiberoptic communication, advanced directional sensors and novel actuators. Photonic crystals may also be used to lower population inversion thresholds and increase solid state device efficiencies. Metallic replicas of photonic crystal structures may be used to provide thermoelectric and heat management devices with the unique ability to operate at precisely-controlled spectral width operations. Soft replicas of photonic crystals have also started to emerge, where the prospects of incorporating them within biological systems has created considerable interest in bio-photonic devices such as sensors, actuators, specialized photon reactors, and the like.
The absence of propagating EM modes inside a photonic band gap (PBG) affords some interesting effects in quantum optics, which may also find practical applications. For example, spontaneous emission of light can be controlled when an excited atom or molecule is embedded within a photonic crystal. The guest molecule can not make a transition to a lower energy state because the emission frequency is within the PBG of the host crystal, increasing the lifetime of its excited state.
As would be understood by one of ordinary skill in the art, if the stop bands for all directions of propagation overlap in some frequency range, a complete PBG can be created in which the density of photonic states (DOS) is zero. Structures exhibiting full PBGs in the microwave, millimeter, and submillimeter regimes have already been fabricated. However, until the present invention, the technological challenge of fabricating a 3-D photonic crystal having ordered structure with respect to the repeat of its subunits (that is, “opal-like”) with controlled and well-defined defects in the visible and near-IR, where many future applications lie, has not been surmounted (1).
The current efforts to realize PBG structures at submicron length scales (necessary for visible and near IR applications) fall in two categories:
i) E-beam lithography/reactive ion etching followed by additional semiconductor growth or wafer fusion.
ii) Self-ordering of submicron colloidal spheres made either from polymers or silica, to form opal-like crystals (also called “synthetic opals”).
E-beam lithography/reactive ion etching suffers from fabrication complexity. Further structures formed by E-beam lithograph/reactive ion etching typically exhibit finite thickness of the high refractive index semiconductor material that results in significant absorption in the visible and near IR region.
Self-ordering of submicron colloidal spheres made either from polymers or silica, to form opal-like crystals offers the advantage of permitting infiltration of the void spaces in the opals with high refractive index material. Synthetic opals made by such technique have been shown to display PBG structures if properly infiltrated with high refractive index materials forming hollow shell structures of the interconnected voids (1, 2). In general, synthetic opals are composed of equal diameter amorphous SiO2 spheres, closely packed in 3D face-centered cubic (fcc) lattices. Stop bands have been observed mostly in the (111) plane direction (See 32 of FIG. 5A), which possesses the maximum scattering efficiency.
Beyond the problem of generating a complete photonic band gap in a synthetic opal formed by self-ordering of submicron colloidal spheres made either from polymers or silica, one of the biggest limitations associated with this approach is the fact that it is extremely difficult to fabricate a very large size single crystal. The process of making a photonic crystal by the self-ordering of submicron monodispersed (less than 5% standard deviation) spheres currently requires slow sedimentation which is typically accompanied by slow solvent evaporation. This sedimentation and evaporation leads to multiple nucleation and growth of polycrystalline photonic materials. In practice, nucleation sites having different crystal orientations (such as illustrated in FIGS. 8A and 8B) have limited technological applications.
Although the recent interest in photonic 3D periodic crystals that exhibit periodic modulation in refractive index has caused significant excitement in the scientific community, commercial realization of such well-controlled periodic structures, in particular where well-defined defects are inserted within, has been elusive. That is, the creation of long-range 3D single crystals with controlled-defects, in a manner that can be mass-produced, is not presently available for most materials. The methods set forth above for manufacturing phototonic crystals are simply too laborious and are limited to a restricted number of materials which can be employed in the construction process.
In light of the efforts of numerous groups around the world working with photonic crystals, there is an unanswered need for fabrication of long range photonic crystals structured as large photonic single crystals with well-defined defects. No economically viable alternative approaches have been reported.